High spatial resolution infrared ellipsometer

ABSTRACT

The invention concerns an ellipsometer comprising a source (S) supplying at least an infrared radiation, a sample-holder (PE), a sensor (D), a first optical system mounted between the source (S) and the sample-holder (PE), so as to illuminate a sample placed on the sample-holder, under oblique view with a polarised light beam and a second optical system mounted between the sample-holder (PE) and the sensor (D) for collecting the light reflected by the sample. The ellipsometer further comprises a blocking device (F 2 ) mounted on the reflection path in the focal plane of the focusing device (M 2 ) of the second optical system, and adapted to block parasite rays (RP) derived from the rear surface (FAR) of the sample and to allow through useful rays (RU) derived from the front surface (FAV) of the sample towards the sensor (D), thereby enabling to obtain a resolution with respect to the sample front and rear surfaces.

[0001] The invention relates to the field of ellipsometry, and moreparticularly to ellipsometry operating in the infrared.

[0002] It has a general application in any field in which ellipsometryis used, and more particularly in microelectronics, in the opticalcharacterisation of a sample, in the optical control of a surfacetreatment, or in the study of the growth of thin layers, for example ofsemiconductor materials, and of their interfaces. It also has anapplication in the cleaning, polishing and preparation of surfaces, inparticular.

[0003] Ellipsometric measurements can be carried out at a fixedwavelength (monochromatic ellipsometry) or at several wavelengths(spectroscopic ellipsometry).

[0004] Depending upon the wavelength range of the source: ultraviolet,visible, near infrared, infrared, etc., it is possible to attaindifferent properties of the layers, of the materials or to exploredifferent materials.

[0005] In practice, the infrared is generally better adapted than thevisible to attain the voluminal properties of the layers and of thematerials.

[0006] In general, an ellipsometer operating in the infrared comprises:

[0007] a source of light radiation supplying at least one infrared beam;

[0008] a sample holder intended to carry a sample of a given thicknessand comprising a front face and a rear face;

[0009] a detector;

[0010] a first optical system mounted between the source and the sampleholder, and comprising a polariser and a focusing device in order toilluminate the sample placed on the sample holder under obliqueincidence by a beam of polarised light; and

[0011] a second optical system mounted between the sample holder and thedetector and comprising a focusing device and an analyser to collect thelight reflected by the sample.

[0012] Because of the very strict requirements in the manufacture ofsemiconductors there is a need for an ellipsometer having a high spatialresolution and the best possible accuracy of measurement.

[0013] With a transparent or semi-transparent sample of a giventhickness, such as silicon, the rear face of the sample can disrupt theellipsometric measurements by reflecting the stray radiation whichcontaminates the detection and the processing of the useful signal.

[0014] This contamination is difficult to deal with because thereflection coefficient of the rear face of a silicon substrate is notalways known. The absorption coefficient k of the substrate is also notalways known. Interference phenomena can also occur with this rear face.Likewise, phenomena of diffusion and/or of diffraction n infrared canalso occur on this rear face. Moreover the rear face may not be parallelto the front face, which can give rise to additional and unnecessarycalculations because only the measurement of the front face is pertinentfor the user.

[0015] Known solutions exist for elimination of the harmful effectsbrought about by the rear face of the sample, particularly the use ofmechanical means such as means for dulling the rear face which rendersthe specular reflections emitted by the rear face negligible.

[0016] When the sample has a relatively great thickness, it is possibleto separate the radiation emitted by the front face from the radiationemitted by the rear face. However, such a solution can only be achievedfor samples of a great thickness, which limits its application.

[0017] Another known solution consists of taking absorbent samples (thatis to say non-transparent samples, such as highly doped silicon), butthis also limits the application of such a solution.

[0018] The object of the present invention is to remedy these drawbacksand to propose a high spatial resolution ellipsometer which operates inthe infrared in which stray reflections due to the rear face of thesample are eliminated.

[0019] The present invention relates to an ellipsometer device of thetype comprising:

[0020] a source of light radiation supplying at least one infrared beam;

[0021] a sample holder intended to carry a sample of a given thicknessand comprising a front face and a rear face;

[0022] a detector;

[0023] a first optical system mounted between the source and the sampleholder, and comprising a polariser and a focusing device in order toilluminate the sample placed on the sample holder under obliqueincidence by a beam of polarised light; and

[0024] a second optical system mounted between the sample holder and thedetector and comprising a focusing device and an analyser to collect thelight reflected by the sample.

[0025] According to a general definition of the invention, theellipsometer device further comprises a blocking device mounted on thereflection path in the focal plane of the focusing device of the secondoptical system and capable of blocking the stray radiation emitted bythe rear face of the sample and allowing the useful radiation emitted bythe front face of the sample to pass through towards the detector, whichmakes it possible to obtain a separating power with regard to the frontand rear faces of the sample.

[0026] For example, the blocking device is of the type constituted by aslot with adjustable dimensions, cutter with adjustable edge, or thelike.

[0027] According to a first preferred embodiment of the invention, theellipsometer device according to the invention further comprises awidening device mounted on the illumination path and capable of wideningthe illuminating beam on the focusing device of the first system and ofwidening the reflecting beam on the focusing device of the secondoptical system.

[0028] For example, the widening device is of the type constituted by aslot with adjustable dimensions, cutter with adjustable edge, divergentlens, or the like.

[0029] The numerical aperture of the focusing device of the firstoptical system is preferably chosen so as to obtain an illuminating beamof small size on the sample. For example the size of the illuminatingbeam on the sample is less than 40 microns×40 microns in the case of alight source of the laser type.

[0030] For example, the focusing devices of the first optical system aswell as of the second optical system comprise at least one opticalelement belonging to the group formed by concave mirrors (for exampleelliptical, parabolic, spherical, etc.).

[0031] The numerical aperture of the focusing device of the secondoptical system is chosen to separate the beams reflected by the frontand rear faces of the sample.

[0032] According to a second preferred embodiment of the invention theellipsometer device according to the invention further comprises aselector device for angles of incidence, mounted on the reflection pathdownstream of the blocking device according to the direction ofpropagation of the light and capable of selecting, for the measurementsby the detector, only the radiation reflected by the sample underoblique incidence within a predetermined range of angles of incidence.

[0033] The selector device is preferably of the type constituted by aslot with adjustable dimensions, cutter with adjustable edge, or thelike.

[0034] In practice, the light source is of the laser type, operating atterahertz frequencies, or silicon carbide source, filament, plasma, orthe like.

[0035] The polariser of the first optical system is preferably of thetype having a grid, with or without a rotating compensator, assemblywith several polarisers with grids, or the like.

[0036] Likewise, the analyser of the second optical system is of thetype having a grid, with or without a rotating compensator, assemblywith two polarisers with grids, or the like.

[0037] For example, the detector is of the type constituted by amercury-cadmium and/or tellurium cell, liquid nitrogen or the like.

[0038] In practice, the sample holder is of the type constituted by atable which is movable in XYZ and/or in rotation, a suspended sampleholder or the like.

[0039] Other characteristics and advantages of the invention will becomeapparent in the light of the following detailed description and thedrawings, in which:

[0040]FIG. 1 is a general diagram of an ellipsometer operating in theinfrared according to the invention; and

[0041]FIG. 2 shows the separating power of the blocking device accordingto the invention as a function of its position with respect to the lightbeams.

[0042] With reference to FIG. 1, a light source S supplies radiation inthe infrared spectrum.

[0043] For example, the source S is of the silicon carbide type at 1200°K. Its spectral range is from 1.44 to 18 microns.

[0044] As a variant, the light source is of the type constituted by alaser operating at terahertz frequencies, a source with a filament,plasma or the like.

[0045] In spectroscopic ellipsometry, the illumination system includesan interferometer of the Michelson type mounted after the source andbefore the polariser in order to scan the spectral range of theapparatus.

[0046] A sample holder PE is intended to carry a sample ECH of a giventhickness and comprising a front face FAV and a rear face FAR.

[0047] The sample is for example a silicon substrate with a thickness ofthe order of 400 to 700 microns.

[0048] The sample holder can be a table which is movable in XYZ and/ormovable in rotation.

[0049] The sample holder can equally be a suspended sample holder.

[0050] Between the source S and the sample holder PE there is provided afirst optical system comprising a polariser P and a focusing device M1.This first optical system makes it possible to illuminate the sample ECHplaced on the sample holder PE, under oblique incidence by a beam ofpolarised light.

[0051] A widening device F1 is advantageously mounted on theillumination path. It can be placed upstream or downstream of thepolariser according to the direction of propagation of the light. Thiswidening device widens the illuminating beam on the mirror M1.

[0052] In practice, the widening device F1 is of the type constituted bya slot with adjustable dimensions, cutter with adjustable edge,divergent lens, or the like.

[0053] In practice, the numerical aperture of the mirror M1 is chosen soas to obtain an illuminating beam of small size on the sample.

[0054] For example, the size of the illuminating beam on the sample isless than 40 microns×40 microns within the framework of a laser source.

[0055] In practice, the mirror M1 which constitutes the focusing deviceof the first optical system is an elliptical mirror.

[0056] As a variant, this element M1 can be a parabolic mirror, aspherical mirror, a lens or even a dioptric, catadioptric optical unitor the like.

[0057] The polariser P is of the type constituted by a polariser with agrid with or without a rotating compensator. As a variant this polarisercan comprise an assembly with two polarisers with grids or the like.

[0058] A second optical system is mounted between the sample holder PEand a detector D. This second optical system comprises a focusing deviceM2 and an analyser A to collect the light reflected by the sample.

[0059] The numerical aperture of the focusing device M2 of the secondoptical system is chosen so as to separate the beams reflected by thefront face FAV and the rear face FAR of the sample.

[0060] The focusing device of the second optical system M2 comprises anoptical element belonging to the group formed by concave mirrors(elliptical, parabolic or spherical), lenses and dioptric orcatadioptric optical units or the like.

[0061] For example, the effective numerical aperture of the focusingdevice M2 of the second optical system is of the order of 2.5°.

[0062] The analyser A of the optical system is of the type constitutedby a polariser with a grid with or without rotating compensator,assembly with two polarisers with grids, or the like. This analyser A isdisposed on the reflecting path downstream of the second mirror M2.

[0063] The detector D is of the type constituted by amercury-cadmium-tellurium cell, liquid nitrogen or the like. Thedetector is compatible with operation in the infrared.

[0064] Between the mirror M2 and the detector D there is advantageouslyprovided another mirror M3 capable of focusing the reflecting beam ontothe detector. The mirror M3 can be of the same type as the mirror M2.

[0065] A selector device for angles of incidence F3 is advantageouslycoupled to the mirror M3. This selector device F3 makes it possible toselect, for the measurements by the detector D, only the radiationreflected by the sample under oblique incidence within a predeterminedrange of angles of incidence.

[0066] For example, the selector device F3 is of the type constituted bya slot with adjustable dimensions, cutter with adjustable edge, or thelike.

[0067] According to the invention a blocking device F2 is provided whichis mounted on the reflecting path in the focal plane of the focusingdevice M2 of the second optical system. This blocking device F2 iscapable of blocking the stray radiation RP emitted by the rear face FARof the sample and allowing the useful radiation RU emitted by the frontface FAV of the sample to pass through towards the detector D.

[0068] Such a blocking device F2 makes it possible to obtain aseparating power with regard to the front and rear faces FAV and FAR ofthe sample.

[0069] The blocking device F2 is advantageously of the type constitutedby a slot with adjustable dimensions, cutter with adjustable edge, orthe like.

[0070] The selector device F3 is advantageously positioned in front ofthe mirror M3 because if it were placed in front of the mirror M2 thediffractions due to the selector device F3 would be such that they woulddegrade the separating power of the blocking device according to theinvention.

[0071] In a preferred embodiment of the invention an optical fibre isdisposed between the source S and the first optical system P. Likewiseanother optical fibre is disposed between the second optical system M3and the detector D.

[0072]FIG. 2 shows intensity curves C1 and intensity derivative curvesC2 of the beam as a function of the position of the cutter F2 withrespect to the normal of the beam.

[0073] The curves C1 and C2 demonstrate the effective separation of thebeams from the front face and the rear face of the sample.

[0074] The present applicants have observed that when an ellipsometeraccording to the diagram as described in FIG. 1 is used, theellipsometer has a separating power with regard to the front and rearfaces of the sample which has a quite satisfactory value in order tocarry out ellipsometric measurements in infrared on semiconductormaterials such as silicon.

[0075] For example, the separating power is of the order of 400 micronswith a light source with a wavelength of the order of 12 microns, asilicon substrate with a thickness of the order of 500 microns, a mirrorM1 with an effective numerical aperture of 2.5°, a mirror M2 with aneffective numerical aperture of the order of 2.5°, and an image of theslot F1 of the order of 300 microns.

[0076] In practice, the carrying out of the invention depends here uponthe size of the spot, the quality of the optics, the thickness of thesilicon substrate and the angle of incidence. For example, with an angleof incidence of 70° the separating power of the blocking device is ofthe order of 600 microns, with a mirror M2 having a magnification factorof 4.21.

1. Ellipsometer device of the type comprising: a source (S) of light radiation supplying at least one infrared beam; a sample holder (PE) intended to carry a transparent or semi-transparent sample (ECH) of a given thickness and comprising a front face and a rear face (FAV and FAR), a detector (D), a first optical system mounted between the source (S) and the sample holder (PE), and comprising a polariser (P) and a focusing device (M1) in order to illuminate the sample placed on the sample holder under oblique incidence by a beam of polarised light, a second optical system mounted between the sample holder (PE) and the detector (D) and comprising a focusing device (M2) and an analyser (A) to collect the light reflected by the sample, characterised in that it further comprises a blocking device (F2) mounted on the reflection path in the focal plane of the focusing device (M2) of the second optical system and capable of blocking the stray radiation (RP) emitted by the rear face (FAR) of the sample and allowing the useful radiation (RU) emitted by the front face (FAV) of the sample to pass through towards the detector (D), which makes it possible to obtain a separating power with regard to the front and rear faces of the sample.
 2. Device as claimed in claim 1, characterised in that it comprises a selector device (M3, F3) for angles of incidence, mounted on the reflection path downstream of the blocking device according to the direction of propagation of the light and capable of selecting, for the measurements by the detector, only the radiation reflected by the sample under oblique incidence within a predetermined range of angles of incidence.
 3. Device as claimed in claim 2, characterised in that the selector device (M3, F3) is of the type constituted by a slot with adjustable dimensions, cutter with adjustable edge, or the like.
 4. Device as claimed in claim 1, characterised in that the blocking device (F2) is of the type constituted by a slot with adjustable dimensions, cutter with adjustable edge, or the like.
 5. Device as claimed in claim 1, characterised in that it further comprises a widening device (F1) mounted on the illumination path and capable of widening the illuminating beam on the focusing device (M1) of the first system and of widening the reflecting beam on the focusing device (M2) of the second optical system.
 6. Device as claimed in claim 5, characterised in that the widening device (F1) is of the type constituted by a slot with adjustable dimensions, cutter with adjustable edge, divergent lens, or the like.
 7. Device as claimed in claim 1, characterised in that the numerical aperture of the focusing device (M1) of the first optical system is chosen so as to obtain an illuminating beam of small size on the sample.
 8. Device as claimed in claim 7, characterised in that the size of the illuminating beam on the sample is less than 40 microns×40 microns.
 9. Device as claimed in claim 1, characterised in that the focusing device (M1) of the first optical system comprises at least one optical element belonging to the group formed by concave mirrors, spherical mirrors, lenses, dioptric or catadioptric optical units or the like.
 10. Device as claimed in claim 1, characterised in that the numerical aperture of the focusing device (M2) of the second optical system is chosen to separate the beams reflected by the front and rear faces of the sample.
 11. Device as claimed in claim 1, characterised in that the focusing device (M2) of the second optical system comprises at least one optical element belonging to the group formed by concave mirrors, lenses, dioptric or catadioptric optical units and the like.
 12. Device as claimed in claim 8, characterised in that the effective numerical aperture of the focusing device (M2) of the second optical system is of the order of 2.5°.
 13. Device as claimed in claim 1, characterised in that the light source (S) is of the type constituted by a laser, terahertz, globar, filament, plasma, or the like.
 14. Device as claimed in claim 1, characterised in that the polariser (P) of the first optical system is of the type constituted by a polariser having a grid, with or without a rotating compensator, assembly with two polarisers with grids, or the like.
 15. Device as claimed in claim 1, characterised in that the analyser (A) of the second optical system is of the type constituted by a polariser having a grid, with or without a rotating compensator, assembly with two polarisers with grids, or the like.
 16. Device as claimed in claim 1, characterised in that the detector (D) is of the MCT type or the like.
 17. Device as claimed in claim 1, characterised in that the sample holder (PE) is of the type constituted by a table which is movable in XYZ and/or in rotation, a suspended sample holder or the like.
 18. Device as claimed in claim 1, characterised in that it comprises an optical fibre between the source (S) and the first optical system.
 19. Device as claimed in claim 1, characterised in that it comprises an optical fibre between the second optical system and the detector (D).
 20. Device as claimed in claim 1, characterised in that the sample (ECH) is of the type constituted by a substrate made from semiconductor material such as silicon. 